The invention disclosed and claimed herein generally pertains to an improved method for acquiring spatially and spectrally localized magnetic resonance (MR) images. More particularly, the invention pertains to a method of such type which corrects or compensates for undesirable effects caused by a perturbation magnetic field, which is produced by an oscillatory slice-selection gradient required for data acquisition.
As is well known by those of skill in the arts relating to MR imaging, a sequence known as a spatial-spectral (SPSP) pulse sequence, or SPSP pulse, can be used to excite a selected spectral species, in a spatially localized region. That is, the SPSP pulse sequence selects a particular slice through a subject (spatial localization) and at the same time selects material lying within the slice which has a particular spectral frequency range, such as water or fat. Such pulse sequences have been found to be especially useful in fast imaging sequences, for suppressing fat signals. Use of the SPSP pulse in an MR imaging sequence is described in further detail in the prior art, such as U.S. Pat. No. 4,999,580, issued Mar. 12, 1991 to Meyer et al.
As is further well known, an SPSP pulse sequence includes a slice-selection gradient field G(t), which oscillates as the SPSP pulse is being played out, so that two-dimensional (k,t)-space can be traversed to simultaneously achieve spatial and spectral localization. The SPSP pulse sequence further includes an RF pulse, comprising a succession of RF sub-pulse components having an envelope defined by the peaks of respective sub-pulses. The shape of the RF sub-pulses and the gradient determine the slice profile. If an MR image is to be acquired from a slice offset by a distance z from the iso-center of an associated MR system (i.e., the center of the MR main magnet), the SPSP pulse has an associated frequency modulation function f.sub.z (t) related to the gradient G(t), for an ideal arrangement, by the expression f.sub.z (t)=.gamma.G(t)z/2.pi., where .gamma. is the gyromagnetic ratio for a given spin species, e.g., protons. It is noted that since the gradient oscillates, the frequency modulation function f.sub.z (t) oscillates as well.
Unfortunately, the SPSP pulse sequence typically results in an arrangement which is not ideal. The oscillatory gradient, required for use with the SPSP pulse as stated above, often produces undesirable effects, such as eddy currents and non-linear gradient amplifier response. These effects, in turn, generate perturbation magnetic fields, which result in considerable signal intensity loss for large values of z, that is, for slices distant from the MR magnet iso-center.